Why not just spend $149 and buy one off Ebay?....just checked and there are some that say are "excellent" condition....seems the way to go. By the time you buy the material, cut it and install it you'd be better off just buying one.
Check this out....
https://www.ebay.com/itm/Porsche-Boxster-Roll-Bar-Wind-Screen-LEXAN-OEM-97-04-EXCELLENT/174411581410?hash=item289bbd87e2:g:~TcAAOSwl5hfSDO O